Breathing insufficiencies, irregularities and other disorders may occur in conjunction with or as a result of a variety health related disorders and may further cause or exacerbate health disorders.
Disordered breathing may contribute to a number of adverse cardiovascular outcomes such as hypertension, stroke, congestive heart failure, and myocardial infarction. Sleep-related breathing disorders, especially central sleep apnea, have been found to have a relatively high prevalence in patients with congestive heart failure and may have a causative or influencing effect on such heart failure. In about 50% of patients with stable congestive heart failure, there is an associated sleep disordered breathing, predominantly central sleep apnea, with a minority having obstructive sleep apnea. Furthermore, sleep related breathing disorders are believed to be physiologically linked with congestive heart failure. Central sleep apnea is a known risk factor for diminished life expectancy in patients who suffer from congestive heart failure. It is also believed that in view of this link, treatment aimed at relieving sleep related breathing disorders may improve cardiovascular outcomes in patients with congestive heart failure.
Pulmonary edema, a condition in which there is excess fluid in the lungs and often found in heart failure patients, is believed in some circumstances to lead to hyperventilation and hyperoxia or apnea. Most heart failure patients with central sleep apnea, when lying flat, tend to have central fluid accumulation and pulmonary congestion, which may stimulate vagal irritant receptors in the lungs to cause reflex hyperventilation.
Breathing disorders may include, for example, hyperventilation, hypoventilation, apnea, and other related breathing disorders. Hyperventilation, which results in hyperoxia, is a condition in which the respiratory rate is pathologically high or is above a desired rate. Hyperventilation may occur due to pulmonary edema or excess fluid built up in the lungs and may ultimately result in apnea episodes. Hypoventilation is a condition in which the respiratory rate is pathologically low or below a desired rate. Apnea (absence of breathing) is a breathing disorder most typically occurring during sleep that can result from a variety of conditions.
Sleep breathing disorders include two types of sleep apnea: obstructive sleep apnea (partial apnea or obstructive apnea) and central sleep apnea. Obstructive sleep apneas result from narrowing of the pharynx with out-of-phase breathing in an effort to create airflow, whereas central sleep apnea arises from reductions or other abnormalities in central respiratory drive. Partial apnea may be a condition in which central respiratory drive is reduced and in which there is an obstruction or narrowing of the airway. During obstructive sleep apnea, respiratory effort increases. Complete apnea or central apnea is defined as a condition where there is no effective EMG signal or phrenic nerve signal, i.e. where there is no effective or significant physiological response. In central sleep apnea, respiratory movements are absent or attenuated but in phase with normal breathing. Sleep apnea typically results in some sort of arousal or wakefulness following cessation of breathing.
Central Sleep Apneas usually are initiated by reduction in PCO2 resulting from an increase in ventilation. When PCO2 falls below the threshold level required to stimulate breathing, the central drive to respiratory muscles and airflow cease or diminish significantly and apnea (or attenuated breathing) ensues until the PCO2 rises again above the threshold required to stimulate ventilation. Often spontaneous arousal occurs with apnea.
Prior to central sleep apnea and some partial apneas, a respiration pattern or breathing disorder called Cheyne-Stokes, frequently occurs. A Cheyne-Stokes respiration pattern of breathing is also frequently associated with congestive heart failure, pulmonary edema, as well as with both obstructive and central apneas. The Cheyne-Stokes respiration pattern comprises a cyclical and often repeating pattern of a period of hyperventilation typically followed by a period of apnea. The hyperventilation portion of a Cheyne-Stokes respiration pattern has a morphology of a gradually increasing breathing pattern in depth and frequency followed by a gradually decreasing pattern in depth and frequency. The period of characteristic hyperventilation is followed by a period of apnea or hypopnea. It is believed that the onset of Cheyne-Stokes in CHF and pulmonary edema patients is caused by low blood oxygen saturation (SaO2) levels resulting from cardiac, pulmonary insufficiencies, or circulatory delay. The SaO2 receptors associated with the carotid artery are believed to perceive a drop in SaO2 where there is a time lag in receiving oxygenated blood due to cardiac insufficiencies.
To compensate for drop in SaO2, hyperventilation typically ensues. And, after a period, this hyperventilation pattern increasing in depth and frequency, the PCO2 levels begin to drop below the chemoreceptor threshold, resulting in apnea.
Currently a number of methods are used to treat sleep apnea. For example, supplemental oxygen supplied through a nasal ventilator has been used to relieve symptoms of sleep apnea. Non-invasive airway pressure including continuous positive airway pressure (CPAP), bivalve and adaptive pressure support servo-ventilation have been used to treat central sleep apnea and obstructive sleep apnea with varying results. Another method to treat central sleep apnea is using aggressive diuresis to lower cardiac filling and beta-blocker and angiotensin-converting enzymes. However, this treatment is often not optimal since excessive use of diuretics leads to renal complications and patient discomfort.
A method and apparatus for treatment of obstructive sleep apnea and central sleep apnea has been proposed where an implantable pulse generator stimulates a nerve in the upper airway tract of a patient and a stimulator stimulates the diaphragm. The upper airway tract stimulation opens the airway tract to permit breathing in response to detecting a pressure differential across the upper airway passage indicating obstructive sleep apnea. The diaphragm stimulation treats central sleep apnea in response to sensing central sleep apnea.
Phrenic nerve stimulation has been used to stimulate the diaphragm throughout an overnight period to treat patients with Central Congenital Hyperventilation at night. The device used was turned on at night to stimulate the nerve continuously and then turned off during the day. However, this device was not adapted for situations where patients would breathe spontaneously. It also does not adjust for a patient's varying needs during the night.
While some methods have been proposed to determine when sleep apnea has occurred and to attempt to induce breathing when apnea occurs, they do not prevent an apnea episode. They also do not predict an apnea episode and therefore can only deliver treatment based on sensed rather than predicted conditions or events.
U.S. Pat. No. 6,641,542 discloses a method and apparatus to detect and treat sleep respiratory events. A processor extracts an average cycle length and frequency of at least one of a Cheyne-Stokes respiration and periodic breathing based upon the physiological data, and determines whether therapy is required based on the average cycle length and the frequency. The specific therapy disclosed was either pacing the heart to increase the heart rate or stimulating the hypoglossal nerve to cause contralateral extension of the patient's tongue or an increase in volume of the patient's oropharynx in accordance with U.S. Pat. No. 5,591,216. While according to the disclosure, the severity of Cheyne-Stokes is used to determine whether or not to stimulate, it is not believed to be used to determine or predict the likelihood of an apnea event subsequently occurring.
Accordingly it would be desirable to provide a method and apparatus for to prevent a breathing disorder episode. It would also be desirable to provide a method an apparatus for identifying a likelihood of a disordered breathing episode occurring.